A number of experimental and theoretical efforts for producing hydrazine based on direct synthesis of N2 and H2 as well as preparation by nitrogen fixation have been undertaken during the past several decades. Most consisted of attempts to stimulate inert H2 into bonding with N2. At best, however, the end product was ammonia. Traces of N2H4 were detected in some cases, but the yield was in the fractional percent range and did not warrant commercial exploitation. Stimulation of the N2—H2 reaction by glow discharge, β-radiation, and α-radiation has also been tried, but no practical results were obtained.
Some experimental efforts were made by Olin et al [J. Chem. Eng. Data, 6, 384 (1991)] under conditions of extreme pressure (667 MPa) and temperature (8000 K), such as those achieved for a few milliseconds in a ballastic piston apparatus containing N2 and H2. This method also failed to produce N2H4 from its elements. This is due to the fact that N2 is a very inert molecule, making it very difficult to form any chemical compound directly from N2. As will be explained later, the temperatures (˜8000 K) turn out to be not high enough to overcome the activation energy requirement. Also the confinement time is too short for the reaction to go to completion. Nevertheless, the energy differentials involved in reductive fixation of N2 have shown that a catalytic process is possible under mildly reducing conditions only. It requires the cooperation of powerful reactants with an absolute potential energy of not less than 1.4 eV. Other than a catalytic approach, however, one based on laser chemistry is also workable as described below.
In order to overcome the difficulties of N2H4 formation by means of direct nitrogen fixation, an energy of activation to promote elementary chemical reactions must be considered for practical applications; that is, in order that the reaction may take place, the stored energy of the system must be raised a certain amount above that of the initial state, or in other words, a potential barrier must be overcome in going from the initial to the final states. This barrier can be high for reactions between saturated molecules; however, it can also be high for reactions in which radicals or free atoms take part, as will be demonstrated for N2+2 H2=N2H4 that is considered below.
Activation energies were first treated theoretically as an application of quantum mechanics by London [Z. Elektrochem. 35, 552(1929)]. Based on knowledge of the potential curves of the participating diatomic molecules, it is possible to predict the approximate magnitude of the activation energy. A quantitative determination can then be made using the experimental data of enthalpy of formation. In this connection, one may mention that diazene, HN═NH, is a short-lived intermediate of N2H4 decomposition and possibly also an intermediary of hydrazine formation through N2 fixation. It is important to know its heat of formation in order to calculate the heat of reactions involving N2H2 as it forms hydrazine. A standard enthalpy of formation for the trans isomer has been measured in a mass spectrometer and calculated from appearance potentials,(ΔH)f298=(212±8) KJ/mol[see Foner et al, J. Chem. Phys. 68, 3162 (1978), ibid, P. 3169–3171; Frost et al, J. Chem. Phys. 64, 4719 (1976)]. Some of the earlier reported enthalpy data were those of Willis et al [Can. J. Chem. 54, 1 (1976); ibid, 47, 3007 (1969)], who prepared N2H2 by microwave discharge in a hydrazine vapor. Their data resulted in a heat of formation of diazene, of(ΔH)f298=(151±8) KJ/molThe ionization potential of N2H4 in their tests was found to be (9.7±0.1) eV, and the ionization potential of the N2H radical was 7.6 eV.
Another contribution to this question is the work by Wiberg, et al [Z. Naturf. 34B, 1385(1979)] who calculated the heat of formation of trans-diazene from ionization and appearance potentials of the molecule fragment to be 134 KJ/mol. The enthalpy of isomerization to iso-diazene is 54 KJ/mol. The N—N and N—H dissociation energies are 510 KJ/mol and 339 KJ/mol, respectively. The energetic relationships between N2H4 and its dehydrogenation products are shown schematically below

This is the energy scheme of dehydrogenation of hydrazine with hydrogen extracted as H2. Diazene is thus in analogy to N2H4, an endothermic compound, and it is expected to decompose spontaneously.
The above considerations then imply
which, in turn, implies that the activation energy for N2H4 formation from N2 and two hydrogen molecules is 0.99 eV. A comparable amount of energy can be stored in N2 as vibrational energy. In order to have a quantitative description we follow the customary formulation where the energy levels of a diatomic molecule may be written as
                                                                                          E                                      v                    ,                    J                                                  =                                                      E                    vib                                    +                                      E                    rot                                                                                                                                            E                  vib                                =                                                                            ω                      e                                        ⁡                                          (                                              v                        +                                                  1                          /                          2                                                                    )                                                        -                                                            ω                      e                                        ⁢                                                                                            x                          e                                                ⁡                                                  (                                                      v                            +                                                          1                              /                              2                                                                                )                                                                    2                                                        +                  …                                                                                                                          E                  rot                                =                                                                            B                      v                                        ⁢                                          J                      ⁡                                              (                                                  J                          +                          1                                                )                                                                              -                                                                                    D                        v                                            ⁡                                              [                                                  J                          ⁡                                                      (                                                          J                              +                              1                                                        )                                                                          ]                                                              2                                    +                  …                                                                    }                            (        1        )            whereBv=Be−αe(v+½)+ . . .Dv=De+ . . .Here v and J are the vibrational and rotational quantum numbers, respectively. In this customary formulation, the constants ωe, Be, etc., as tabulated for diatomic molecules, are given in units of cm−1. For N2, we haveωe=2358.57 cm−1 ωexe=14.32 cm−1 Be=1.99824 cm−1 αe=0.017318 cm−1 De=5.76*10−6 cm−1To convert cm−1 to eV, one uses 1 eV=8068.3 cm−1. For a vibrational quantum number v=4, we haveEv=4,J=0−Ev=0,J=0=1.13 eV>0.99 eVwhich is larger than the required activation energy, and one should expect that

A method of obtaining N2(v=4) from N2(v=0) begins when N2(v=0) is pumped to N2(v=8) by two-photon absorption with high energy, short pulse Nd: YAG lasers (λ=1.06 μm) providing the photons. As will be shown below, under intense laser irradiation, half of the N2 will end up in the v=8 state, while the other half will remain in the v=0 state when the two-state up and down transitions between v=0 and v=8 are saturated. If this excited gas of N2 is subsequently compressed at high pressures (˜100 Atm), then because of near-resonant V—V energy transfers an energy equilibrium will be reached when all N2 molecules are in the v=4 state, and no further energy exchanges between these molecules are possible. When N2(v=4) gas mixes with H2 gas with the molar ratio of 1:2 as in (B) under the conditions of high gas pressure and low temperatures (˜300 K), liquid hydrazine will be formed.
One may point out that the physical concept just described is in fact a very common practice in the scientific community. The most prominent example is excimer lasers, such as XeCl and KrF. As is well known, Xe and Kr are inert gases like He, Ne, and Ar; they don't react with other atoms in their ground states. However, when Xe or Kr is pumped to an excited state, it becomes chemically active, and will react with Cl and F to form bound diatomic molecules XeCl and KrF, respectively. Thus the failure to entice N2 to react with H2 can be understood to occur because certain energy conditions are required that are not properly met. My proposed methods to satisfy the required energy condition will thus have important practical consequences.
There remains an important scientific issue concerning how reaction (B) takes place. In addition to energy considerations and the relationship that (A) is a reverse process to (B), a number of intermediate chemical processes occur before the final product, which is N2H4, forms as in reaction (B). From the known molecular structure of N2H4, namely,

The intermediate step in reaction (B) must therefore involve an electron transfer process. The experimental evidence as well as the theoretical basis for hydrazine electron transfer (ET) chemistry have been extensively established by a number of investigators. The following review articles and papers contain the scientific information that underlies and supports the present invention:                (a) S. F. Nelson, “Molecular Structure and Energetics”. In Liebman et al, Eds.; VCH Publishers, Inc., Deerfield Beach, Fla., 1986, Vol. 3, Chapter 1;        (b) S. F. Nelson, Acc. Chem. Res. 14, 131 (1981); and        (c) K. Kobuta et al, J. Phys. Chem. 86, 602 (1982).        
Finally, I wish to point out that hydrazine is in many respects very much like water. The following table compares several important physical properties of those two substances. Also included are the boiling points of H2, N2 and O2. They are, of course, very different from those of N2H4 and H2O. It appears that hydrazine is suitable to replace gasoline for internal combustion engine if the price is right.
H2ON2H4H2N2O2Molecular Weight, g/mol183222832Boiling point, ° C.100113.5−258−196−183Melting point, ° C.01.8Liquid density, g/cm310.997Vapor pressure, Torr (mmHg)2510.4Dielectric constant78.351.7